1. Field of the Invention
The present invention pertains to a magnetic fluid composition which can be used in shaft sealing applications, in particular, for vacuum apparatus, hard disk drive devices for computers, etc. which require high standards of cleanliness, a process for making the same and a magnetic fluid sealing device produced therewith.
2. Description of the Prior Art
In a computer hard disk, the hard disk which functions as a magnetic recording device is disposed in a casing and is rotationally driven by a motor located outside of the casing. Since the hard disk should be strictly kept from contamination caused by dusts or the like, a sealing means is disposed at the region in which the driving shaft of the motor passes through the wall surface of the casing to inhibit the intrusion of the dusts.
Various remedies have been proposed to eliminate this problem. For purposes of discussion, they can be grouped in three classifications or examples. In the first Example, it is known to employ magnetic fluid seal as a sealing means for hard disk drives, as shown in FIG. 6. In this prior art application, a shaft bearing 3 for supporting a drive shaft 2 for a hard disk carrying flange 1 is fitted to a housing 4. Two pole pieces 6 and 7 having a permanent magnet 5 located therebetween are secured to the inner circumferential surface of the housing 4 made of a nonmagnetic material such as aluminum or a synthetic resin. The drive shaft 2 passing through the pole pieces 6 and 7 is made of a magnetic material, and a narrow gap 9 is formed between the outer circumferential surface 2A of the shaft 2 and the inner circumferential surfaces 6A and 7A of the pole pieces 6 and 7. A magnetic fluid 10 is filled in the gap 9 and held by a magnetic force to thereby completely seal the inside from the outside of the casing.
The conventional magnetic fluid 10, when it is used as the sealing agent described above, is usually prepared by using a hydrocarbon oil, that is, a less volatile organic solvent such as mineral oil, synthetic oil, ester oil or ether oil as a dispersing medium (carrier) and dispersing fine ferromagnetic particles coated a the surface thereof with a dispersing agent having oleophilic groups that have an affinity with the hydrocarbon oil stably present in the dispersing medium.
Examples of suitable dispersing media include, poly-alpha-olefin oils, alkyl naphthanalene oils, hexadecyldiphenylether,octadecyldiphenyletherandtri-2-ethylhexyl trimellitate. The dispersing agent can be a surface active agent, such as synthetic sulfonic acids, polybutene succinic acid, polybutene sulfonic acid, unsaturated fatty acids, phosphate esters and the salts thereof.
Magnetic fluids comprising hydrocarbon oil as the carrier exhibit good "wetting" characteristics. When used in a sealing device, these materials will wet the outer surface of the drive shaft, such as surface 2A of the drive shaft in FIG. 6. The fluid tends to extend over the wide range of the outer surface 2A of the drive shaft. If the magnetic fluid 10 extends over the wide range of the outer surface 2A of the drive shaft, the amount of the magnetic fluid capable of providing the aimed sealing function is decreased relatively to shorten the working life of the sealing device.
Further, if the magnetic fluid 10 extending along the outer surface 2A of the drive shaft 2 also extends along the lateral side 6B of the pole piece of the inner circumferential surface 4A of the housing 4, the magnetic fluid can be scattered by centrifugal force into the clean space in which the hard disk is disposed. If deposited on the hard disk, the magnetic fluid would cause malfunction in the computer, etc.
A countermeasure for excessive "wetting" characteristics described above is discussed in Japanese Patent Publication Sho 60-48668 which discloses a structure such as that shown in FIG. 7, and, for purposes of this discussion is referred to as the second Example in which a membrane 11 is attached to the outer surface 2A of the driving shaft 2 and the lateral side 6B of the pole piece, near the gap between shaft 2 and the pole pieces 6 and 7. The material may be made of a fluorocarbon such as polytetrafluoroethylene (PTFE). In this device, it is intended to effectively prevent "wetting" with the magnetic fluid 10 by directly coating the outer surface of 2A of the drive shaft and the lateral side 6B of the pole pieces with the membrane of the fluorocarbon material while taking notice of the oil repellent property thereof.
It has also been known to formulate a magnetic fluid using a non-volatile and oil repellent perfluoropolyether oil as a carrier, in which fine ferromagnetic particles are disposed (see U.S. Pat. No. 3,784,471 and Japanese Laid Open Patent Application Sho 61-263202). The fluorocarbon magnetic fluid disclosed in those references is intended for use in specific conditions such as in corrosive atmospheres or under low temperatures in which the use of a magnetic fluid comprising a simple hydrocarbon oil carrier is difficult. Because the fluorocarbon magnetic fluid exhibits diminished "wetting" characteristics with metal, diffusion to the outer surface of the drive shaft or the lateral sides of the pole pieces may be reduced when used in the sealing device. Accordingly, hard disk contamination due to the "wetting" can also be decreased (Example 3).
However, the above-mentioned second example of prior art involves the following problems:
(1) Since fluorocarbon compounds such as polytetrafluoroethylene (PTFE), etc. are expensive and exhibit poor molding characteristics, the cost for forming the necessary coating membrane is expensive.
(2) Since it is difficult to control the thickness of the coating membrane to ensure proper thinness and uniformity, the size of the gap between the outer surface 2A of the driving shaft and the inner circumferential surfaces 6A and 7A of the pole pieces can be irregular. Because of this the required amount of magnetic fluid cannot be retained. This results in shortening the life span of the sealing mechanism.
(3) If the coating membrane of PTFE experiences peeling, it forms relatively large dust flakes that can float in the clean space enhousing the hard disk. The PTFE dust particles can adhere to the disk surface, etc. resulting in improper operation in the associated computer or other similar apparatus.
(4) If the coating membrane is peeled or the coating surface contains rough or injured portions, the magnetic fluid tends to extend along the recesses of such portions resulting in the contamination of the clean space from the scattering of the magnetic fluid.
In the third prior art example given, if the magnetic fluid containing the perfluoropolyether oil as a carrier is used for sealing the rotational shaft, it causes the following problems:
(1) The perfluoroether oil used as the carrier is itself expensive. Furthermore, the surface active agent used as the dispersing agent for the fine ferromagnetic particles is also expensive since its hydrophobic groups have a perfluoropolyether structure.
(2) When the fluorocarbon magnetic fluid is used for the dust-proof sealing device, for example in computers, hydrocarbon oil mists formed at the outside of the disk housing such as oil mists generated from the shaft bearing 9 for supporting the driving shaft 2 intrude into the fluorocarbon magnetic fluid. The hydrocarbon oil has lower specific gravity as compared with the fluorocarbon oil and is incompatible with it. Accordingly, the hydrocarbon oil mists are not trapped in the sealing portion but easily intrude into the clean space of the casing by phase separation to contaminate the inside of the latter. Such "phase separation" also occurs, for example, in cases where fluorocarbon magnetic fluids are used to establish the vacuum seal of a vacuum device which has a shaft bearing for which a hydrocarbon oil is used as the lubricant. This results in contamination of the inside of the vacuum vessel.
Because of the reasons described above, magnetic fluids using perflouropolyether oil as the carrier are exclusively used in specific circumstances such as liquid seals or corrosive gas seals in which the use of a magnetic fluid containing an ordinary hydrocarbon oil is not preferred. Use of such materials is not suitable to accomplish dust-proof sealing for computers or vacuum sealing and, as such, lacks in general applicability.
It is, accordingly, an object of the present invention to provide a magnetic fluid composition which does not suffer from peeling or injury, exhibits lowered characteristics of wetting to metal, and does not shorten seal life span due to diffusion, scattering or evaporation, as well as being less expensive, convenient to use, and able to prevent contamination of the sealed space due to phase separation which can occur with hydrocarbon lubricants.
Another object of the present invention is to provide a process for producing the magnetic fluid composition as described above.
A further object of the present invention is to provide a sealing device using the magnetic fluid composition as described above.